1. Field of the Invention
The present invention relates to the preparation of diesters of hexene-1,6-dioic acids. Such diesters can be facilely hydrogenated into the corresponding diesters of adipic acid, or adipates, which can in turn be hydrolyzed to form adipic acid. Adipic acid, one of the raw materials for nylon 66, is currently produced in vast amounts and, because of this fact alone, any novel synthesis for this diacid and/or derivatives thereof would be of fundamental interest.
The present invention especially relates to the preparation of diesters of 3-hexene-1,6-dioic acid by reacting carbon monoxides with at least one 1,2-dialkoxy-3-butene in the presence of a palladium-based catalyst.
2. Description of the Prior Art
It is known to this art, per Imamura and Tsuji, Tetrahedron. vol. 25, p. 4187-4195 (1969), to prepare diesters of 3-hexene-1,6-dioic acid by reacting carbon monoxide with 1,4-diethoxy-2-butene in ethanol, in the presence of palladium and chloride.
1,2-Diethoxy-3-butene is mentioned as a coproduct, assumed to originate by an allyl rearrangement of 1,4-diethoxy-2-butene.
U.S. Pat. No. 4,611,082 describes dicarbonylating a 1,4-dialkoxy-2-butene in a polar, aprotic and nonbasic solvent, at 80.degree. to 140.degree. C. in the presence of a transition metal halide, palladium chloride being the most effective.
This same type of reaction, starting with 1,4-dimethoxy-2-butene, is described in detail in Journal of Molecular Catalysis, 53. pp. 417-432 (1989).
Over the course of research by the assignee hereof relating to the preparation of dialkoxybutenes from 1,3-butadiene, it has been shown that, in general, a mixture is produced containing especially the 1,4-dialkoxy-2-butene (predominant ether) and the 1,4-dialkoxy-3-butene, these two diethers being relatively difficult to separate from each other.